J . Phys. Chem. 1989, 93, 10-12
10
Relation between Electrical Percolation and Rate Constant for Exchange of Material between Droplets in Water in Oil Microemulsions A. Jada, J. Lang,* and R. Zana Institut Charles Sadron (CRM- EAHP). CNRS- ULP Strasbourg 6, rue Boussingault, 67083 Strasbourg Cedex, France (Received: August 29, 1988) The rate constant k, associated with the exchange of material upon collisions between droplets in ternary water in oil microemulsions stabilized by ionic surfactants has been determined by a time-resolved fluorescence method. The electrical conductivity of the same systems has been investigated. It appears that percolative conduction occurs only in systems where k, is larger than (1-2) X lo9 M-’ 8. This result supports the hypothesis that above percolation threshold the conductivity is mainly due to the motion of counterions through water channels or fusion between droplets in droplet clusters.
Introduction Water in oil (w/o) microemulsions generally consist of small water droplets surrounded by a surfactant monolayer and dispersed in an oil-rich continuous phase. In recent years there have been a large number of studies’-5 of w/o microemulsions which aimed at determining droplet sizes, interactions between droplets, and, in some cases, rate constants which characterize these systems such as exchange of material between droplets upon “sticky” droplet collisions,6 acccrding to .the reaction k,
when, for instance, the volume fraction of the dispersed phase (water surfactant) or the temperature is increased. These changes have been attributed to the occurrence of a percolation transition.” The percolation threshold corresponds to the formation of the first infinite cluster of droplets. The number of such clusters increases very rapidly above percolation threshold giving rise to the observed changes of properties, in particular, to the increase of electrical conductivity discussed here. The electrical conductivity above percolation threshold volume fraction or temperature has been attributed to either “hopping” of surfactant ions from droplet to droplet within droplet clustersi3J8or transfer of counterions from one droplet to another through water channels” opening between droplets during “sticky” collisions or through transient merging of droplet^.'^ However, no correlation was reported so far between the Occurrence of a percolation transition in microemulsions and the values of k,. We show below that values of k, larger than (1-2) X lo9 M-’ s-l are required for the occurrence of percolative conduction in a number of w/o microemulsions, including microemulsions whose electrical conductivity has been measured by other authors. It is hoped that this result will stimulate new theoretical studies of percolation in these systems.
+
The droplet size depends on the molar ratio w = [H20]( [surfactant] as well as on the temperature and nature of the oil. The interactions between water droplets have been found to be more or less attractive depending on the nature of the cosurfactant and oil. The concept of “sticky” droplet collisions6 has been naturally associated with these attractive interactions. The second-order rate constant k, which characterizes the exchange of material between droplets through “sticky” collisions (see reaction 1) has been determined for several s y ~ t e m ~and ~ ~ found - ’ ~ to be as large as 5 X lo9 M-’s-l, in some instances. Experimental Section On the other hand, electrical conductivity studies have shown We have studied two types of ternary (water-oil-surfactant) that a large and steep increase of the conductivity (by several orders of magnitude) occurs in some w/o m i c r o e m ~ l s i o n s ~ ~ ~ J ~ -w/o ’ ~ microemulsions: (i) waterln-alkanes (hexane to dodecane)/AOT (sodium bis(Zethylhexy1) sulfosuccinate); (ii) water/chlorobenzene/cationic surfactants (alkylbenzydimethyl(1) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. ammonium chlorides, referred to as Nm,Bz,l,l with m = 12, 14, Biophys. Acta 1988, 947, 209 and references therein. (2) Cazabat, A. M.; Langevin, D. J . Chem. Phys. 1981, 74, 3148. and 16, synthesized as part of this work). (3) Chatenay, D.; Urbach, W.; Cazabat, A. M.; Langevin, D. Phys. Reu. The first systems have been well characterized. In particular Lett. 1985, 54, 2253. we have recently reported k, values as a function of temperature (4)Lang, J.; Jada, A.; Malliaris, A. J . Phys. Chem. 1988, 92, 1946. and oil chain length.4 Some of these data will be used here to (5) Huang, J. S. J . Chem. Phys. 1985, 82, 480. (6) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. 1981, show the correlation in relation with percolation. Droplet sizes 85, 863. and interactions between droplets in systems ii have been inves(7)Brochette, P.; Pileni, M. P. Nouu. J . Chim. 1985, 9, 551. Pileni, M. tigated and will be reported elsewhere.*O P.; Furois, J. M.; Hickel, B. In Surfactants in Solution; Mittal, K., Lindman, The electrical conductivity was measured using an automated B., Eds.; Plenum: New York, 1984;Vol. 3, p 1471. (8) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J . Chem. Sac., autobalanced conductivity bridge (Wayne-Kerr B 905) operating Faraday Trans. 1 1987, 83, 985. at 1 kHz. (9)Howe, A. M.; McDonald, J. A,; Robinson, B. H. J . Chem. SOC., The rate constants k, for exchange of material between droplets Faraday Trans. 1 1987, 83, 1007. (10)Lianos, P.; Zana, R.; Lang, J.; Cazabat, A.-M. In Surfactants in were determined by the time-resolved fluorescence quenching Solufion; Mittal, K., Bothorel, P., Eds.; Plenum: New York, 1986;Vol. 6, method4smwith ruthenium(I1) tris(bipyridine) ions as fluorescence p 1365. probe and ferricyanide and methylviologen ions as quenchers in (11) Lagiies, M.; Ober, R.; Taupin, C. J. Phys. Lett. 1978, 39, L-487. the case of microemulsions based on AOT and cationic surfactants, Lagiles, M.; Sauterey, C. J . Phys. Chem. 1980, 84, 3503. respectively. The probe was selected for its long lifetime and its (12) Eicke, H. F.; Hilfiker, R.; Holz, M. Helu. Chim. Acta 1984,67, 361. (13) Hilfiker, R.;Eicke, H. F.; Geiger, S.; Furler, G. J. Colloid Interface solubility in water. All chemicals were the same as in previous Sci. 1985, 105, 378. s tudie~.~,~~ (14)Geiger, S.;Eicke, H. F. J. Colloid Interface Sci. 1986, 110, 181. (15) Van Dijk, M. A.; Casteleijn, G.; Joosten, J. G. H.; Levine, Y. K. J. Results Chem. Phys. 1986,85, 626. (16) Borkovec, M.; Eicke, H. F.; Hammerich, H.; Gupta, B. D. J. Phys. Figures 1 and 2 show the variations of the rate constant k, and Chem. 1988, 92, 206. the electrical conductivity K , as a function of w for w/o micro(17) Mathew, C.; Patanjali, P. K.; Nabi, A,; Maitra, A. Colloids Surf. 1988, 30, 253.
(18) Bhattacharya, S.;Stockes, J. P.; Kim, M. W.; Huang, J. S. Phys. Reu. Lett. 1985, 55, 84. (19)Dutkiewicz, E.; Robinson, B. H. J . Electroanal. Chem., in press. I
,
.,
(20) Jada, A.; Lang, J.; Zana, R.; Makhlofi, R.; Hirsch, E.; Candau, S. J. Manuscript in preparation.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 11
Letters
00
0 - 1
5
IW
25 w 35
0 20 Lo 60 w 80 Figure 1. Variations of rate constant k, and electrical conductivity K with 15
for the water/AOT/n-heptane (0) and water/AOT/n-decane (+) microemulsions. Variation of the conductivity, K, with w for the water/AOT/n-octane ( 0 )microemulsions. The arrow and value on each curve indicate the volume fraction of dispersed phase for onset of electrical percolation. Surfactant concentration,0.182 M; temperature, w
25
OC.
Figure 3. Variations of rate constant, k, (X) and electrical conductivity, K (+) with temperature for the water/AOT/n-decane microemulsions with w = 26.3. Surfactant concentration, 0.182 M.
Figure 2. Variations of rate constant k, and electrical conductivity K with w for the water/chlorobenzene/Nm,Bz,l,lmicroemulsions, for m = 12 ( 0 ) ,m = 14 (+), and m = 16 ( 0 ) .The arrow and value on each curve indicate the volume fraction of dispersed phase for onset of electrical percolation. Surfactant concentration, 0.27 M; temperature, 20 OC.
emulsions stabilized by AOT and by cationic surfactants, respectively. The maxima of K seen at low w are due to two antagonistic effects: first, an increasing hydration of surfactant ion-counterion contact ion pairs with w which results in an increase of electrical conductivity; second, an increasing size of water droplets with w, which results in a decrease of their concentration and mobility and thus of K. The second effect becomes predominant at high w when most ion pairs are hydrated. The large and steep increases of K seen in Figures 1 and 2 at high w , for some systems, indicate the occurrence of percolative conduction. For the AOT-stabilized microemulsions percolation occurs with n-octane and n-decane but not with the shorter oil n-heptane, at 25 OC. Moreover, the percolation threshold is seen to decrease as the oil chain length is increased. The results of Figure 2 for the microemulsions based on cationic surfactants Nm,Bz,1,1 show that the effect of the surfactant chain length is opposite to that of the oil chain length. There is no percolation for the Iong-chain surfactant N16,Bz,l,l whereas the shorter chain surfactants N14,Bz,l,l and N12,Bz,l,1 show a percolation transition. Besides, the percolation threshold decreases with the surfactant chain length. Figures 1 and 2 show that k, increases with w for all of the investigated systems. However, at a given w, k, increases rapidly
upon increasing oil (alkane) chain length and decreasing surfactant chain length. Moreover, most importantly, the results of Figures 1 and 2 reveal that percolation takes place upon increasing w only when k, has reached a sufficiently high value: about (1-2) X lo9 M-' s-'. For the water/AOT/n-heptane and water/ N 16,Bz, 1,l /chlorobenzene systems, the k, values remain below lo8 M-' s-' , under the experimental conditions used, and percolation does not take place, even at very high w. The requirement that k, must be large enough, around (1-2) X lo9 M-' s-', for the Occurrence of percolation appears to apply to a wide variety of w/o microemulsions. A few examples are given below from our own results and/or results reported by others. (i) Figure 3 shows the variation of K and k, as a function of temperature (T) for the water/AOT/n-decane microemulsion at a fixed w = 26.3. Percolation is seen to take place a t above 30 OC,when k, becomes larger than 1.5 X lo9 M-' s-'. (ii) As part of this work, we have found for the water/ N16,Bz,111/benzene microemulsions k, values larger than 1.5 X lo9 M-' s-' for w > 10. On the other hand, Chatenay et al.3 have reported the Occurrence of percolation in this same system. Notice that the results of Figure 1 showed low k, values and no percolation for the water/N16,Bz,l,l/chlorobenzene system. Thus the substitution of chlorobenzene by benzene is sufficient to induce a very large increase of k, and, in turn, the occurrence of percolation. (iii) Geiger and Eicke14 have reported the occurrence of percolation as the temperature of water/AOT/isooctane system is increased. The extrapolation of their data yields a threshold temperature of 47 OC at w = 26. The corresponding value for k, was found to be (1-2) X lo9 M-' (iv) For the system water/AOT/n-decane with w = 25 Dutkiewicz and R o b i n ~ o n reported '~ a threshold percolation temperature of 17 OC. Our previous results4 for the same system with w = 26.3 indicate that k, = 1.5 X lo9 M-' s-' at T = 17 OC. The above results clearly show that electrical conductivity percolation occurs whenever the rate constant k, for droplet collision with exchange of material becomes larger than (1-2) X lo9 M-' s-l, irrespective of the nature of the surfactant (anionic, cationic) and oil (alkane, aromatic) temperature and volume fraction of the dispersed phase. Discussion Volume Fraction of Dispersed Phase at the Percolation Threshold in w/o Microemulsions. The volume fraction of dispersed phase (droplets with their surfactant layer) required for the occurrence of percolation is known to be close to 0.15.11,21 However, the above results show that volume fractions ranging (21) Safran, S.A.; Webman, I.; Grest, G. S.Phys. Rev. A 1985, 32,506.
J . Phys. Chem. 1989, 93, 12-14
12
between 0.05 (water/N12,Bz, 1,l/chlorobenzene in Figure 3) and 0.24 (water/N14,Bz,l,l/chlorobenzenein Figure 2) can be found for the percolation threshold, depending on the nature of the system. The results indicate that the value of the percolation threshold is decreased as the strength of the attractive interdroplet interactions increases, as predicted by recent theoretical calculations.2'p22 Indeed, the virial coefficient for the droplet translational diffusion coefficient determined by quasielastic light scattering has been found to become increasingly negative, indicating increasingly attractive interactions as the surfactant chain length decreases for the water/chlorobenzene/Nm,Bz, 1,l microemulsionsZo and as the oil chain length increases for the water/AOT/n-alkane m i c r o e m u l s i ~ n s . ~From ~ ~ ~ the above it is also clear that a strong correlation exists between the variations of k, and interdroplet interactions with surfactant and oil chain lengths. Mechanism of Electrical Conductivity above Percolation Threshold in w/o Microemulsions. Various mechanisms have been proposed to explain the percolative conduction observed with some w/o microemulsions, the most accepted one being associated (22)Safran, S. A.;Grest, G. S.; Bug, A.L.R.In Microemulsion Systems; Rasano, H., Clause, M., Eds.; Surfactant Science Series; Dekker: New York, 1987;p 235. (23) Hou,M. J.; Kim, M.; Shah, D. 0.J. Colloid InterfaceSci. 1988,123, 398.
with surfactant ions hopping from one droplet to the neighboring ones in the clusters of droplets then present in the microemulsion above percolation threshold. Such a mechanism, however, does not easily explain why percolation occurs only when the rate constant for droplet collisions with exchange of material becomes larger than (1-2) X lo9 M-' s-l. In fact this feature suggests that the electrical conductivity is mainly due to the motion of counterions in the narrow water tubes or channels created within the clusters of droplets present above the percolation threshold upon opening of the surfactant layers separating water cores of contiguous droplets. The length and number of these transient tubes and in turn the conductivity would increase rapidly above the percolation threshold.
Conclusions We have shown that the rate constant k , for droplet collision with exchange of material must reach a value of a least (1-2) X lo9 M-' s-l for percolative conduction to occur in a number of w/o microemulsions, irrespective of the parameter which is varied to induce percolation and the nature of the w/o microemulsion. We have also shown that the increase of k , and the occurrence of electrical percolation correlate with an increase of attractive interactions between droplets. We are currently undertaking experiments on more complex w/o microemulsions to check the degree of generality of these conclusions.
Adsorptlon and Dlssociation of CH&I on Clean and Potassium-Promoted Pd( 100) Surfaces Andras Berko and Frigyes Solymosi* Reaction Kinetics Research Group of Hungarian Academy of Sciences and Institute of Solid State and Radiochemistry, University of Szeged, P.O. Box 105, H-6701 Szeged, Hungary (Received: October 6, 1988)
CH3C1adsorbs molecularly on a clean Pd( 100) surface at 100 K and desorbs intact without a detectable dissociation. Preadsorbed potassium dramatically influences these processes: it induces the cleavage of C-CI bond to yield adsorbed CH,and C1 even at 135 K which then react further to give CH4, C2H6,C,, and Hz.
Introduction The study of the adsorption and dissociation of alkyl halides on metal surfaces has a strong relevance to the better understanding of C1 chemistry, particularly the synthesis of methane and methanol from C O and C 0 2 as well as the partial oxidation of methane. The adsorption and thermal dissociation of CH3Cl Pt( 11 1),3 has so far been investigated on Fe(l00),' Ni( Al( 1 1 1): and Ag( 11 1)5 single-crystal surfaces. The adsorption was weak and molecular; CH3CI dissociated only on Fe(100). Recently, we found that the potassium adatom is an excellent activator for C 0 2on Pd(100) and Rh(l.11) surfaces.@ It induced (1) Benzinger, J. B.; Madix, R. J. J. Catal. 1980, 65, 49. White, J. M. Surf. Sci. 1988, 194, 438. (2) Zhou, X.-L.; (3) Hederson, M. A.;Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184,
L32.5. (4) Chen, J. G.; Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Am. Chem. SOC.1987, 109, 1726. ( 5 ) Zhou, X.-L.; Blass, P. M.; Cannon, K. C.; Solymosi, F.; White, J. M. Surf. Sci., submitted for publication. (6) Solymosi, F.; Berko, A. J . Catal. 1986, 101, 458. (7) Solymosi, F.;Bugyi, L. J. Chem. SOC.,Faraday Trans. 1 1987, 83, 2015.
0022-3654/89/2093-0012$01.50/0
a change in the structure and bonding of COz and led to its dissociation. The key compound in the interaction is the COzanion. Potassium was also an effective promotor for other compounds, such as C 0 , 9 CH30H,l0 and HCOOH," playing an important role in Cl reactions. In this work we present the first study of the effect of an alkali-metal overlayer on the adsorption and reaction of CH3C1 on a transition-metal surface. It is demonstrated that potassium as an electron donor strongly influences the stability of CHJCl molecule, which desorbs practically intact from most of the transition metals. Experimental Section Experiments were performed in a standard UHV chamber equipped with facilities for A B , EELS (in the electronic range), TDS,and work function measurements. Sample preparation, cleaning, and potassium deposition have been described in our previous paper.l2 (8) Kiss, J.; Revesz, K.; Solymosi, F. Surf. Sci., in press. (9) Solymosi, F.; Berko, A. Surf. Sci. 1988, 201, 361. (10)Solymosi, F.;Tarnoczi, T.; Berko, A. J. Chem. Phys. 1987,87, 6745. ( 1 1) Solymosi, F.; Kiss, J.; Kovacs, I. J. Phys. Chem. 1988, 92, 796.
0 1989 American Chemical Society
